METAR

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting.

METARs typically come from airports or permanent weather observation stations. Reports are generated once an hour or half-hour, but if conditions change significantly, a report known as a special (SPECI) may be issued. Some METARs are encoded by automated airport weather stations located at airports, military bases, and other sites. Some locations still use augmented observations, which are recorded by digital sensors, encoded via software, and then reviewed by certified weather observers or forecasters prior to being transmitted. Observations may also be taken by trained observers or forecasters who manually observe and encode their observations prior to transmission[citation needed].

The METAR format was introduced 1 January 1968 internationally and has been modified a number of times since. North American countries continued to use a Surface Aviation Observation (SAO) for current weather conditions until 1 June 1996, when this report was replaced with an approved variant of the METAR agreed upon in a 1989 Geneva agreement. The World Meteorological Organization's (WMO) publication No. 782 "Aerodrome Reports and Forecasts" contains the base METAR code as adopted by the WMO member countries.[1]

In addition, a short period forecast called a TREND may be added at the end of the METAR covering likely changes in weather conditions in the two hours following the observation. These are in the same format as a Terminal Aerodrome Forecast (TAF).

The complement to METARs, reporting forecast weather rather than current weather, are TAFs. METARs and TAFs are used in VOLMET broadcasts.

METAR code is regulated by the World Meteorological Organization in consort with the International Civil Aviation Organization. In the United States, the code is given authority (with some U.S. national differences from the WMO/ICAO model) under the Federal Meteorological Handbook No. 1 (FMH-1), which paved the way for the U.S. Air Force Manual 15-111[5] on Surface Weather Observations, being the authoritative document for the U.S. Armed Forces. A very similar code form to the METAR is the SPECI. Both codes are defined at the technical regulation level in WMO Technical Regulation No. 49, Vol II, which is copied over to the WMO Manual No. 306 and to ICAO Annex III.

Although the general format of METARs is a global standard, the specific fields used within that format vary somewhat between general international usage and usage within North America. Note that there may be minor differences between countries using the international codes as there are between those using the North American conventions. The two examples which follow illustrate the primary differences between the two METAR variations.[6][7]

041600Z indicates the time of the observation. It is the day of the month (04) followed by the time of day (1600 Zulu time, which equals 4:00 pm Greenwich Mean Time or 6:00 pm local time).

12012MPS indicates the wind direction is from 120° (east-southeast) at a speed of 12 m/s (23 knots; 27 mph; 44 km/h). Speed measurements can be in knots (abbreviated KT) or meters per second (abbreviated MPS).

NOSIG is an example of a TREND forecast which is appended to METARs at stations while a forecaster is on watch. NOSIG means that no significant change is expected to the reported conditions within the next 2 hours.

8849//91 indicates the condition of the runway:

88 indicates either a specific runway (e.g. 25 = Rwy 25 or 25L; adding 50 will indicate Right Runway) or all the airport's runways ("88").

99 indicates repetition of the last message as no new information received.

Some locations will report the runway using 3 characters (e.g. 25L)

4 means the runway is coated with dry snow

9 means 51% to 100% of the runway is covered

// means the thickness of the coating was either not measurable or not affecting usage of the runway

91 means the braking index is bad, in other words the tires have bad grip on the runway

CAVOK is an abbreviation for Ceiling And Visibility OK, indicating no cloud below 5,000 ft (1,500 m) or the highest minimum sector altitude and no cumulonimbus or towering cumulus at any level, a visibility of 10 km (6 mi) or more and no significant weather change.[8]

North American METARs deviate from the WMO (who write the code on behalf of ICAO) FM 15-XII code. Details are listed in the FAA's Aeronautical Information Manual (AIM), but the non-compliant elements are mostly based on the use of non-standard units of measurement. This METAR example is from Trenton-Mercer Airport near Trenton, New Jersey, and was taken on 5 December 2003 at 18:53 UTC.

051853Z indicates the day of the month is the 5th and the time of day is 1853 Zulu/UTC, 6:53PM GMT, or 1:53PM Eastern Standard Time.

04011KT indicates the wind is from 040° true (north east) at 11 knots (20 km/h; 13 mph). In the United States, the wind direction must have a 60° or greater variance for variable wind direction to be reported and the wind speed must be greater than 3 knots (5.6 km/h; 3.5 mph).

P0002 indicates that 0.02 inches (0.5 mm) of liquid-equivalent precipitation accumulated during the last hour.

T10171017 is a breakdown of the temperature and dew point in eight digits separated into two groups of four. The first four digits (1017) indicate the temperature. The first digit (1) designates above or below zero Celsius (0=above zero 1=below zero). The next three digits in the group "017" give the temperature in degrees and tenths of a degree Celsius, −1.7 °C (28.9 °F). The last four digits "1017" indicate the dew point, −1.7 °C (28.9 °F). Note: ASOS software, as of this update, uses whole degrees in °F to compute the °C values in this group.[citation needed]

= indicates the end of the METAR.

In Canada, RMK is followed by a description of the cloud layers and opacities, in eighths (oktas). For example, CU5 would indicate a cumulus layer with 5⁄8 opacity.[12]

METARs can be expressed concisely using so-called aviation flight categories, which indicates what classes of flight can operate at each airport by referring to the visibility and ceiling in each METAR. Four categories are used in the U.S.:[16]

METAR abbreviations used in the WX section. Remarks section will also include began and end times of the weather events.[17]

Codes before remarks will be listed as "-RA" for "light rain". Codes listed after remarks may be listed as "RAB15E25" for "Rain began at 15 minutes after the top of the last hour and ended at 25 minutes after the top of the last hour."

6 hour maximum temperature. Follows RMK with five digits starting with 1. Second digit is 0 for positive and 1 for negative. The last 3 digits equal the temperature in tenths.

This example value equals −23.4 °C (−10 °F).

20123

6 hour minimum temperature. Follows RMK with five digits starting with 2. Second digit is 0 for positive and 1 for negative. The last 3 digits equal the temperature in tenths.

This example value equals 12.3 °C (54 °F).

4/012

Total snow depth in inches. Follows RMK starting with 4/ and followed by 3 digit number that equals snow depth in inches.

This example value equals 12 inches of snow currently on the ground.

402340123

24-hour maximum and minimum temperature. Follows RMK with nine digits starting with 4. The second and sixth digit equals 0 for positive for 1 for negative. Digits 3–5 equal the maximum temperature in tenths and the digits 7–9 equals the minimum temperature in tenths.

This example value equals 23.4 °C (74 °F) and 12.3 °C (54 °F).

52006

3 hour pressure tendency. Follows RMK with 5 digits starting with 5. The second digit gives the tendency. In general 0–3 is rising, 4 is steady and 5–8 is falling. The last 3 digits give the pressure change in tenths of a millibar in the last 3 hours.

3 or 6 hour precipitation amount. Follows RMK with 5 digits starting with 6. The last 4 digits are the inches of rain in hundredths. If used for the observation nearest to 00:00, 06:00, 12:00, or 18:00 UTC, it represents a 6-hour precipitation amount. If used in the observation nearest to 03:00, 09:00, 15:00 or 21:00 UTC, it represents a 3-hour precipitation amount.

This example shows 1.23 inches (31 mm) of rain.

70246

24-hour precipitation amount. Follows RMK with 5 digits starting with 7. The last 4 digits are the inches of rain in hundredths.

This example shows 2.46 inches (62 mm) of rain.

8/765

Cloud cover using WMO code. Follows RMK starting with 8/ followed by a 3 digit number representing WMO cloud codes.

98060

Duration of sunshine in minutes. Follows RMK with 5 digits starting with 98. The last 3 digits are the total minutes of sunshine.

This example indicates 60 minutes of sunshine.

931222

Snowfall in the last 6-hours. Follows RMK with 6 digits starting with 931. The last 3 digits are the total snowfall in inches and tenths.

This example indicates 22.2 inches (560 mm) of snowfall.

933021

Liquid water equivalent of the snow (SWE). Follows RMK with 6 digits starting with 933. The last 3 digits are the total inches in tenths.